1. Field of the Invention
The present invention relates to a light source device capable of switching the angle range of irradiation, to a display device provided with this light source device and capable of switching the angle range of visibility, to a terminal device equipped with this display device, and to an optical member used in the aforementioned light source device.
2. Description of the Related Art
Because of their thin profile, light weight, small size, low energy consumption, and other advantages, display devices that use liquid crystals have been widely deployed and used in a range of devices that includes monitors, televisions (TV: Television), and other large terminal devices; notebook-type personal computers, cash dispensers, vending machines, and other mid-sized terminal devices; and personal TVs, PDAs (Personal Digital Assistance: personal information terminal), mobile telephones, mobile gaming devices, and other small terminal devices. These liquid crystal display devices can be generally classified as transmissive, reflective, or transflective (using transmitted light and reflected light jointly) according to the type of light source used. Energy consumption can be reduced in the reflective type, since it can utilize external light in the display, but contrast and other aspects of display performance are inferior compared to the transmissive type. Therefore, transmissive and transflective liquid crystal display devices are currently in the mainstream. In transmissive and transflective liquid crystal display devices, a light source device is installed on the back surface of a liquid crystal panel, and a display is created using the light emitted by the light source device. Specifically, a light source device that is separate from the liquid crystal panel is essential in current mainstream liquid crystal display devices.
In the liquid crystal panel that is the primary component of a liquid crystal display device, information is displayed by using an electric field to control the orientation of liquid crystal molecules, but numerous modes have been proposed according to the combination of the type and initial orientation of the liquid crystal molecules, the direction of the electric field, and the like. Among these modes, the modes most often used in a conventional terminal device include an STN (Super Twisted Nematic) mode using a simple matrix structure, and a TN (Twisted Nematic) mode using an active matrix structure. However, a liquid crystal panel that uses these modes has a narrow range of angles in which contrasts can be correctly distinguished, and grayscale inversion occurs outside the optimum viewing position.
This problem of grayscale inversion was relatively insignificant in mobile telephones and other terminal devices when the display content consisted mainly of telephone numbers and other characters. However, with recent technological development, terminal devices have come to display not only text information, but also large amounts of image information. The visibility of images is therefore severely reduced by grayscale inversion. Liquid crystal panels that use a mode having a wide range of angles at which contrast can be correctly distinguished without the occurrence of grayscale inversion are therefore gradually being installed in terminal devices. Liquid crystal panels having this type of mode are referred to generically as wide-viewing-angle liquid crystal panels, and in-plane switching systems and other in-plane modes, multi-domain vertical alignment modes, and the like are applied therein. Since gradation can be correctly distinguished in a wide range of angles by using these wide-viewing-angle liquid crystal panels, even though a medium-sized terminal device is basically a personal tool, applications that can be appreciated by multiple people simultaneously and are designed for sharing information with others are being developed and gradually installed.
On the other hand, medium-sized terminal devices are characteristically used not only in closed rooms under tight security, but also in public places. It then becomes important to keep displays of private information and confidential information from being viewed by a third party. Particularly in recent years, occasions where private information and confidential information are displayed have increased in conjunction with the development of terminal devices, and demand for eavesdropping prevention techniques is increasing. There is therefore a desire for development of a technique capable of preventing eavesdropping, and enabling the display to be viewed only by the user, by narrowing the range of angles in which the display is visible.
As described above, it is desired to have both a display that has a wide range of viewing angles and can be appreciated by multiple people simultaneously, and a display that has a narrow range of viewing angles and can be viewed only by the user. The ability to switch between these two types of displays in a single terminal device is also desired. Therefore, in order to satisfy such requirements, a display has been proposed in which the light source device essential to the liquid crystal display device is designed so that the range of viewing angles can be changed.
FIG. 35 is a schematic sectional view showing the conventional viewing-angle-controlled liquid crystal display device described in Japanese Laid-Open Patent Application 9-244018; and FIG. 36 is a schematic perspective view showing an illumination device in which this viewing-angle-controlled liquid crystal display device is used. As shown in FIG. 35, the conventional viewing-angle-controlled liquid crystal display device 1101 is composed of a liquid crystal display element 1102; a scatter control element (scatter control means) 1103; and an illumination device (backlight) 1104. The scatter control element 1103 is disposed between the liquid crystal display element 1102 and the illumination device 1104. As shown in FIG. 36, the illumination device 1104 is provided with an opaque slitted sheet (translucent sheet) 1120 and an irradiating unit 1121. A fluorescent tube or other light source 1122 is provided to the irradiating unit 1121, and a reflecting sheet 1124 disposed on the surface facing the light-emitting surface 1123 is provided for reflecting the light from the light source 1122. The surface on the side of the opaque slitted sheet 1120 forms the light-emitting surface 1123 for emitting the light from the light source 1122 and guiding the light to the opaque slitted sheet 1120. In the opaque slitted sheet 1120, a plurality of linear opaque members that extend in one direction are arranged parallel to each other on one surface of a translucent sheet. The direction in which the opaque members extend coincides with the vertical direction of the display unit.
In the conventional viewing-angle-controlled liquid crystal display device configured as described in Japanese Laid-Open Patent Application 9-244018, the light emitted from the light source 1122 is radiated to the scatter control element 1103 via the opaque slitted sheet 1120. When the light emitted from the light-emitting surface 1123 passes through the opaque slitted sheet 1120, the opaque slitted sheet 1120 increases the collimation of the transmitted light. Therefore, there is no incidence of light to the scatter control element 1103 at angles that are significantly tilted with respect to the direction orthogonal to the light-incident surface. Specifically, transmitted light is obtained that is highly parallel to the direction orthogonal to the surface of the opaque slitted sheet 1120. The light emitted from the illumination device 1104 then enters the scatter control element 1103. The scatter control element 1103 controls the scattering properties of the incident light rays according to the presence of an applied voltage. When the scatter control element 1103 is in a scattering state, the light from the illumination device 1104 is scattered by the scatter control element 1103; and when the scatter control element 1103 is in a transmitting state, the light from the illumination device 1104 is not scattered.
In the viewing-angle-controlled liquid crystal display device 1101 configured as described above, when the scatter control element 1103 is in the scattering state, the highly collimated light emitted from the illumination device 1104 is scattered by the scatter control element 1103 and caused to enter the liquid crystal display element 1102. As a result, the light that has passed through the liquid crystal display element 1102 is released in all directions in the viewing angle of the display unit, and it becomes possible to recognize the displayed content also from positions other than the position directly in front of the display unit. In contrast, when the scatter control element 1103 is in the transmitting state, the highly collimated light emitted from the illumination device 1104 is caused to enter the liquid crystal display element 1102 while still maintaining a high degree of collimation, without being scattered by the scatter control element 1103. As a result, the transmitted light of the liquid crystal display element 1102 is not propagated to positions where the display unit is viewed at an angle to the left or right in the horizontal direction, the screen is darkened when viewed from such a position, and it becomes impossible to recognize the displayed content. By this configuration, only an observer who is directly facing the display unit can recognize the displayed content.
As described above, in the viewing-angle-controlled liquid crystal display device 1101 having the abovementioned configuration, the viewing angle characteristics of the displayed content can be controlled because the scattering properties of the light can be controlled by the scatter control element 1103. Specifically, since highly collimated light can be emitted towards the liquid crystal display element 1102 by the illumination device 1104, when the scatter control element 1103 is placed in the transmitting state, viewing angle characteristics can be reliably obtained in which only an observer directly facing the display unit can recognize the displayed content. Consequently, a liquid crystal display device can be obtained that is capable of arbitrarily switching between a state in which display characteristics are uniformly maintained in all viewing angle directions with little dependence on viewing angle, and a state in which the displayed content can be recognized only from a position directly facing the display unit.
A light source device having increased directivity has been investigated in the past. FIG. 37 is a schematic perspective view showing the first conventional high-directivity light source device cited on pages 14 through 21 of the April 2004 issue of Monthly Display. As shown in FIG. 37, the first conventional high-directivity light source device is composed of a light source 2101; an optical waveguide 2102 for propagating and emitting in planar fashion the light emitted by the light source 2101; a diffusing sheet 2103 disposed on the side of the light-exiting surface of the optical waveguide 2102; two prism sheets 2104 and 2105 disposed on the diffusing sheet 2103; a diffusing sheet 2106 disposed on the prism sheets; and a reflecting sheet 2107 disposed on the opposite side from the light-exiting surface of the optical waveguide 2102. A dot shape is printed on the surface of the optical waveguide 2102. A prism shape in a one-dimensional arrangement extending in one direction is formed in the two prism sheets 2104 and 2105. The apex angle of this prism shape is 90 degrees. The prism sheets 2104 and 2105 are also arranged so that the extension direction of the prism shape formed in the prism sheet 2104 and the extension direction of the prism shape formed in the prism sheet 2105 are orthogonal to each other. Furthermore, the prism sheets 2104 and 2105 are arranged so that the prism surfaces face upwards (the side opposite the optical waveguide).
In the first conventional high-directivity light source device that has this type of configuration and is cited on pages 14 through 21 of the April 2004 issue of Monthly Display, the light emitted from the light source 2101 enters the optical waveguide 2102, and is propagated in the optical waveguide 2102. A portion of the light is then scattered by a printed dot pattern and emitted from the emitting surface of the optical waveguide 2102. The uniformity ratio of illuminance of the light emitted from the optical waveguide 2102 is enhanced by the diffusing sheet 2103 disposed between the optical waveguide 2102 and the prism sheet 2104, and the light enters the prism sheets 2104 and 2105. Since the apex angle of the prism sheets 2104 and 2105 is 90 degrees, light rays directed near a 30-degree angle from the front refract and proceed in the front direction. As a result, the light rays are focused in the front direction, and the frontal luminance is enhanced.
FIG. 38 is a schematic perspective view showing the second conventional high-directivity light source device cited on pages 14 through 21 of the April 2004 issue of Monthly Display. As shown in FIG. 38, the second conventional high-directivity light source device is composed of a linear light source 3101; an optical waveguide 3102 for propagating and emitting in planar fashion the light emitted by the light source 3101; a prism sheet 3103 disposed on the side of the light-exiting surface of the optical waveguide 3102; and a reflecting sheet 3104 disposed on the opposite side from the light-exiting surface of the optical waveguide 3102. The optical waveguide 3102 is a matte prism optical waveguide in which a matte pattern shape (not shown in the drawing) is formed on the light-exiting surface thereof, and a row of prisms extending in the direction (hereinafter referred to as the direction orthogonal to the light source) orthogonal to the extension direction of the light source 3101 is formed on the surface of the reflecting sheet 3104 side, which is the surface on the opposite side. The prism sheet 3103 is arranged with the prism surface towards the side of the optical waveguide, and the extension direction of the prism rows is the direction (hereinafter referred to as the direction parallel to the light source) that is parallel to the extension direction of the light source.
In the second conventional high-directivity light source device that has this type of configuration and is cited on pages 14 through 21 of the April 2004 issue of Monthly Display, the light emitted from the light source 3101 enters the optical waveguide 3102 and is propagated in the optical waveguide 3102. A portion of the light is then excluded from the condition of total reflection by the matte pattern formed in the light-exiting surface, which is the surface on the side of the prism sheet of the optical waveguide 3102, and is emitted from the optical waveguide 3102. The light emitted from the optical waveguide 3102 is in a condition slightly removed from the total reflectance condition of the optical waveguide 3102, and is therefore highly directed light having a peak near 65 degrees from the normal to the emitting surface in the direction orthogonal to the light source. This light enters the prism sheet 3103, but is totally reflected by the tilted surface of the prism on the opposite side and emitted in the frontal direction after being refracted by the tilted surface of the prism on the incident side.
As previously mentioned, since the light that is incident on the prism sheet 3103 has high directivity in the direction orthogonal to the light source, the light emitted from the prism sheet also has high directivity with respect to the direction orthogonal to the light source. On the other hand, directivity in the direction parallel to the light source is ensured by forming a row of prisms extending in the direction orthogonal to the light source in the surface on the side of the reflecting sheet 3104 of the optical waveguide 3102.
FIGS. 39A and 39B are graphs showing the results of comparing the directivity characteristics of the second conventional high-directivity light source device with the directivity characteristics of the first conventional high-directivity light source device, wherein the horizontal axis represents the exit angle, and the vertical axis represents the light intensity. FIG. 39A shows the directivity in the vertical direction, and FIG. 39B shows the directivity in the horizontal direction. FIGS. 39A and 39B show what is described in FIG. 14 on pages 14 through 21 of the April 2004 issue of Monthly Display. As shown in FIGS. 39A and 39B, in the second conventional high-directivity light source device, the directivity is increased not only in the direction orthogonal to the light source, but also in the parallel direction, and directivity is increased more than by the first conventional high-directivity light source device.
FIG. 40 is a schematic perspective view showing the third conventional high-directivity light source device described in Japanese Laid-Open Patent Application 2003-215584, and FIG. 41 is a sectional view thereof. As shown in FIG. 40, the third conventional high-directivity light source device is primarily composed of an optical waveguide 4132, a light emitting unit 4133, a reflecting panel 4134, and a diffusing prism sheet 4135. The optical waveguide 4132 is formed in a square flat panel shape from polycarbonate resin, methacrylic resin, or another transparent resin, and an optical diffusion pattern 4136 is formed on the back surface thereof. A light-incident surface 4137 is formed in one location of the corner part of the optical waveguide 4132 by cutting the corner part at an angle, as viewed in a plane. In the light emitting unit 4133, one or more LEDs (Light Emitting Diode: light-emitting diode) are sealed in a transparent molded resin, and the surfaces other than the front surface of the molded resin are covered by a white resin. The light emitted from the LED is emitted from the front surface of the light emitting unit 4133 directly or after being reflected by the interface between the molded resin and the white resin.
This light emitting unit 4133 is disposed in a position in which its front surface faces the light-incident surface 4137 of the optical waveguide 4132. The optical diffusion pattern 4136 formed on the bottom surface of the optical waveguide 4132 is arranged in concentric arcs centered around the light emitting unit 4133 (particularly, the internal LED), and each optical diffusion pattern 4136 is formed in a curve by recessing the rear surface of the optical waveguide 4132 in the shape of an asymmetrical triangle in cross-section. The optical diffusion patterns 4136 also spread out along the circumferential direction of the arc having the light emitting unit 4133 at the center, and the reflecting surfaces of the optical diffusion patterns 4136 intersect with the direction that links the light emitting unit 4133 and the optical diffusion pattern 4136 as viewed in a plane. The optical diffusion pattern 4136 is also formed so that the pattern density gradually increases in the direction away from the light emitting unit 4133. The surface of the reflecting panel 4134 has a mirror finish formed from Ag plating, and is disposed so as to face the entire rear surface of the optical waveguide 4132. In the diffusing prism sheet 4135, a transparent textured diffusion panel 4139 is formed on the surface of a transparent plastic sheet 4138, and a transparent prism sheet 4140 is formed on the rear surface of the plastic sheet 4138.
In the third conventional high-directivity light source device that has this type of configuration and is described in Japanese Laid-Open Patent Application 2003-215584, the light p emitted from the light emitting unit 4133 enters the optical waveguide 4132 from the light-incident surface 4137, as shown in FIG. 41. The light p entering the optical waveguide 4132 from the light-incident surface 4137 spreads in radial fashion in the optical waveguide 4132, but the optical element 4144 formed in the light-incident surface 4137 is designed so that the light intensity in each direction of the light p spreading in the optical waveguide 4132 at this time is proportional to the surface area of the optical waveguide 4132 in each direction. The light p entering the optical waveguide 4132 is propagated within the optical waveguide 4132 in the direction away from the light emitting unit 4133 while repeatedly undergoing total reflection between the top surface and bottom surface of the optical waveguide 4132. The angle at which the light p incident on the bottom surface of the optical waveguide 4132 is incident on the top surface (light-exiting surface 4145) of the optical waveguide 4132 decreases each time the light is reflected by the optical diffusion pattern 4136 having the triangular cross-sectional shape, and the light p that is incident on the light-exiting surface 4145 at an incidence angle smaller than the critical angle of total reflection passes through the light-exiting surface 4145 and exits to the outside of the optical waveguide 4132. Each optical diffusion pattern 4136 is also arranged so as to be orthogonal to the direction linking the light emitting unit 4133 to each optical diffusion pattern 4136. Therefore, even when the light p propagated in the optical waveguide 4132 is diffused by the optical diffusion pattern 4136, the light p is diffused within the plane that is perpendicular to the optical waveguide 4132 and includes the direction linking the light emitting unit 4133 to the optical diffusion pattern 4136, but proceeds straight ahead without being diffused in the plane that is parallel to the tangent of the optical diffusion pattern 4136. The light p that has passed through the bottom surface without being reflected by the top surface of the optical waveguide 4132 is directly reflected by the reflecting panel 4134 facing the bottom surface of the optical waveguide 4132, is returned to the optical waveguide 4132, and is again propagated in the optical waveguide 4132. After the minimally diverged, highly-directed light emitted from the optical waveguide is bent towards the direction perpendicular to the light-exiting surface 4145 by passing through the prism sheet 4140 of the diffusing prism sheet 4135, and is then diffused to the appropriate degree by the textured diffusion panel 4139 of the diffusing prism sheet 4135, the light is emitted in the frontal direction with high directivity. High directivity is therefore achieved.
However, the conventional viewing-angle-controlled liquid crystal display device described above has such problems as the following. Specifically, in the conventional viewing-angle-controlled liquid crystal display device described in Japanese Laid-Open Patent Application 9-244018, the directivity of the light emitted from the irradiation unit of the illumination device is increased by using an opaque slitted sheet to block components that are obliquely incident at angles equal to or greater than a certain angle with respect to the light-incident surface. Therefore, the light emitted from the light source is utilized with low efficiency in this conventional display device. The highly directed light emitted from the opaque slitted sheet enters the scatter control element. In the case of a wide-angle display, the scatter control element is placed in the scattering state, and the high-directivity light is scattered in a wide range of angles. In this display device, since the light having high directivity in the frontal direction that has selectively passed through the opaque slitted sheet spreads in a wide range of angles, the luminance is significantly reduced at all angles, and the visibility of the display is significantly reduced in comparison to a normal illumination device that does not have an opaque slitted sheet.
The inventors conducted a concentrated investigation in order to overcome this problem. As a result, the inventors discovered that the amount of luminous flux transmitted by the opaque slitted sheet (hereinafter referred to as the light-direction regulating element) is effectively increased by increasing the directivity of the light emitted from the light source device (irradiation unit of the illumination device). Transmittance of the light-direction regulating element depends on the angle, so that the transmittance is highest in the frontal direction and gradually decreases as the size of the angle increases. This is because the apparent open area ratio of the opaque slits decreases as the angle increases. Specifically, by increasing the directivity of the light emitted from the light source device, and concentrating the luminous flux in the frontal direction in which the transmittance of the light-direction regulating element is high, the luminous flux passing through the light-direction regulating element can be increased. The directivity of the light emitted from the light source device is more preferably increased in two-dimensional fashion in the in-plane direction of the light-direction regulating element, but the transmitted luminous flux can be increased by at least increasing the directivity in the alignment direction of the opaque slits. The luminance can thereby be enhanced in all directions even when the scatter control element is in the scattering state.
Based on this discovery, the inventors conducted additional investigation of the conventional high-directivity light source device in order to enhance the luminance of the conventional viewing-angle-controlled liquid crystal display device during display in a wide field of view. As a result, it was determined that the enhancement of luminance is inadequate even when the conventional high-directivity light source device is used.
In the first conventional high-directivity light source device cited on pages 14 through 21 of the April 2004 issue of Monthly Display, the luminous flux that is emitted from the optical waveguide and endowed with more uniform luminance by the diffusion sheet is refracted by two orthogonally arranged prism sheets. As a result, the directivity in the frontal direction is increased, but even though the two prism sheets refract light rays directed near 30 degrees from the normal to the frontal direction, light rays at other angles are refracted or totally reflected in directions other than the frontal direction. The increase in directivity characteristics is therefore limited. As a result, the luminous flux transmitted by the light-direction regulating element is not adequately increased, and the luminance during wide-angle display cannot be adequately enhanced.
In the direction perpendicular to the light source in the second conventional high-directivity light source device cited on pages 14 through 21 of the April 2004 issue of Monthly Display, higher directivity can be achieved than in of the first conventional light source since the luminous flux having high directivity emitted from the optical waveguide is emitted in the frontal direction using the total reflection of the prism sheet. Directivity is increased in the direction parallel to the light source by a row of prisms that is provided to the side of the reflecting sheet of the optical waveguide and is directed perpendicular to the light source, but directivity in the direction perpendicular to the light source is low, and the increase in the two-dimensional light condensation properties is limited. Consequently, when this light source device is installed in the display device of a mobile telephone, the direction perpendicular to the high-directivity light source preferably runs to the left and right of the display screen. The light source of the light source device is then disposed on the left and right of the display screen. However, since the chassis is made thinner in a normal mobile telephone, it is impossible to place the light source on the left and right of the display screen. The light source is therefore placed above and below the display screen when the second conventional high-directivity light source device is applied to a mobile telephone, but then it is impossible to increase the directivity in the left/right direction. As a result, the luminous flux transmitted by the light-direction regulating element is not adequately increased, and the luminance during wide-angle display cannot be adequately enhanced.
Furthermore, in the third conventional high-directivity light source device described in Japanese Laid-Open Patent Application 2003-215584, the two-dimensional light focusing properties can be increased by making the light source a point light source, and by using an optical waveguide and diffusion prism sheet in which a pattern of concentric circles is arranged around the light source. By this configuration, the ratio of luminous flux transmitted by the light-direction regulating element can be increased. However, the light source must be concentrated in one location, and the intensity of a normal LED light source is inadequate. The intensity of the light source is increased when a light source is used in which a plurality of LED are housed in a single package, but the integration of the light source is limited by the problem of heat emission. As a result, the luminous flux transmitted by the light-direction regulating element is not adequately increased, and the luminance during wide-angle display cannot be adequately enhanced.